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Lecture 8

HSS1100 Lecture 8

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Department
Health Sciences
Course
HSS1100
Professor
William Yan
Semester
Fall

Description
Lecture 8: Viruses -we don't know as much about viruses as we do bacteria -very hard to grow and see Slide 2: General Characteristics -viruses are not truly cells; too simple to be considered true cells -are either DNA or RNA (never both) surrounded by a protein coating; extremely simple structure -always require a host cell for them to attach to and get into so that they can borrow some of the replication machinery in order to multiply; they are too simple to do this on their own -host cell can be any type of cell (i.e.: bacteria, human cell, animal cell, plant cell, etc.) -it is impossible to grow / culture viruses using artificial media -we need cell tissue cultures in order to grow viruses -cell tissue cultures are much more expensive and more difficult to work with -a virus can produce only either DNA or RNA and never both -it has to find whatever it is missing from the host cell -viruses are extremely small (10 nm to 300 nm) -smaller than prokaryotes and eukaryotes and slightly bigger than proteins -viruses are basically nucleic acids (DNA or RNA) that is protected by an external protein coat which may have some external antigens Replication -there are 5 key steps in viral replication: 1) adsorption -this is a very specific step -the virus must find on the surface of the host cell a receptor that it can recognize; structures on the protein coat bind to the receptor and this initiates replication 2) penentration and uncoating -the virus internalizes itself by crossing the cell membrane and entering the host cell -this happens immediately after adsoprtion -penetration and uncoating happen almost simultaneously -the protein coat breaks apart and the nucleic acid escapes 3) nucleic acid and protein synthesis -the protein coat enters the nucleus of the host cell and integrates itself into the host cells chromosome -it fools the host cell to think that the foreign DNA is part of its own DNA; everytime the host cell replicates DNA, it also replicates the viral DNA and produces viral nucleic acids and viral proteins are then created -the virus fools the host cell into manufacturing parts for the virus (i.e.: nucleic acid, protein, etc.) -protein and nucleic acid builds up and leads to assembly 4) assembly -this step makes a functional virus from the parts created during nucleic and protein synthesis -intact viruses are created -the host cell ends up being packed with the virus and cannot survive and will lyse 5) release -the host cell lyses and releases the functional viruses -each virus that is released starts the whole process again by finding a new host -all viruses that infect humans go through these 5 steps in their lifecycle -the time it takes to complete a cycle determines the incubation period for the virus How do we detect viral infections? -we must use electron microscopes to detect viruses -culturing or detecting viruses are very difficult 1) Detect the virus itself -electron microscopy 2) Detect the immune response (antibodies against the virus) -we look for antibodies that are produced by a person -a high antibody count indicates exposure to the virus specific to that antibody Viral Diagnosis A) Detection in Clinical Specimens 1) Visualization by Electron Microscopy -must be present in large numbers in order to detect 2) Cell Culture -more indirect -look for viruses in tissue culture -when some viruses infects and attaches itself into the host cell, it kills the host cell eventually; you can see a transparent zone on the plate which indicates that the tissue is dead and there are a large number of viruses multiplying -you can indirectly confirm that there is something that is killing the tissue / host cell (i.e.: cytopathic effects) -we can also use immunofluorescence which causes the virus to fluoresce against a black background -we are not seeing the virus itself, but rather a signal that indicates a virus is present -there are viruses that cause blood to clot -we can use hemagglutination tests to indicate if a virus is present -these are all indirect tests to see if there is a possibility of the presence of viruses 3) Detection of Viral Antigens (ELISA) -similar to immunofluorescence -a fluorescent tag is hooked onto an antibody molecule which attaches itself to an antigen -machines are used to detect signals and give a numerical reading in order to determine a positive viral detection -much quicker than traditional immunofluorescence and much more sensitive B) Detection of Patient's Immune Responses -we can also look for an immune response in the patient (this is indirect) 1) Immunity Test -anitgens are prepared and we detect / look for antibodies in the blood of the patient -stronger reactions means more viruses present -indirect detection; we are not actually seeing any viruses 2) Diagnostic Test -the higher the antigen titer, the more antibodies present and the more severe / developed the infection Slide 14: What types of viruses will we learn about? -we will be grouping viruses based on the symptoms -because viruses are not considered true living cells, we often don't use its genus species name 1) Viruses of Breathing 2) Enteric Viruses 3) Viruses of Diarrhea 4) Viruses of the Rashes 5) Viruses of the Glands 6) Viruses of the Liver 7) Viruses of the Brain 8) Viruses of the Immune System (HIV) Slide 15: Respiratory Viruses -the name primary symptoms involve respiratory infections -the following are mainly respiratory viruses (i.e.: a cough) but some of them can cause other secondary and more serious symptoms (on the rare occasion) 1) Influenza Viruses -there are different types of influenza viruses based on the differences in their protein coat -Influenza virus type A is responsible for all the different flu outbreaks around the world (i.e.: swine flu, common flu, etc.) -Influenza virus type B is responsible for milder infections -influenza viruses produce hemagglutinin (responsible for agglutinating RBCs) -the H in H1N1 -neuraminidase (the N in H1N1) -the H and N antigens mutate constantly so there are many possibilities of combinations -pandemics = global pandemics -symptoms include fever, respiratory symptoms, etc. -can do serology tests -look for antibody titer increases -when looking for antibody reactions, you almost always do a paired serum (taking a minimum of 2 samples in a range of 2 to 3 days); if there is an active infection, the antibody level should be higher in the second test and if the second reading is similar or lower than the first, the particular virus that you are testing for may not be causing the infection and you should look for another potential pathogen -treatment for the flu virus is usually not common (typical theme for viral infections) -antiviral drugs are not easily available and are much more expensive than antibiotics -most antiviral drugs have very serious side effects which may be even worse than the original infection -for viral infections, generally treatment is the last resort; we focus more on prevention -we spend more money on vaccination for viruses -the influenza flu vaccine is not the most successful vaccine that we have developed -we have to do annual vaccinations especially for high risk groups -annual vaccinations because of the recombination and constant changing of viruses and its antigens; previous vaccines will never work for the next year because of the constantly changing strains -every year, a scientist determines the best combination of viruses to put into the vaccine -there is a pattern of change that viruses will follow -we can use the seasonal differences between the southern and northern hemispheres (there is a good chance that the strains that caused infections in the southern hemisphere will emerge later in the northern hemisphere) -the vaccine can be very effective one year or very ineffective -viral symptoms are often caused by the damage of the host cells by the virus -often, antiviral drugs will destroy host cells because it cannot differentiate between the virus inside and the host cell -the WHO Pandemic response and planning is a direct reflection of disease spread (how fast it spreads and how widespread) rather than disease severity -the 2009 H1N1 strain involves a combination of 8 segments from different viruses 2) Parainfluenza Viruses -not the same as influenza virus -does not cause outbreaks; causes the rare, individual outbreak -there is no vaccination for this because it is uncommon -causes symptoms like respiratory infections, a croup, etc. 3) Respiratory Syncytial Viruses (RSV) -causes 2 major outbreaks a year (around late fall / early winter and around march / april) -more common in young children / babies ( <2 years of age) -children are usually hospitalized especially if they have chronic lung infections -causes a cough that produces a lot of phlegm -for babies, they cannot clear their throat and are usually lying down so there could be a buildup of secretions and cause serious respiratory issues -use ventolin and a suction to clear / remove secretions -there is now a vaccine available but is not given as a general vaccine -RSV can be t
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